XVIth International Workshop on Quantum Systems in Chemistry and Physics 

Abstract 
Twolayer QM/QM’ calculations for a geometry optimization of large biradical systems 
Yasutaka Kitagawa, Toru Saito, Yusuke Kataoka, Natsumi Yasuda, Hiroshi Hatake,
Toru Matsui, Takashi Kawakami, Shusuke Yamanaka, Mitsutaka Okumura, Kizashi Yamaguch
Graduate School of Science, Osaka University, Japan 
With the recent progress in quantum chemistry, we can calculate electronic structures, energies and energy derivatives of large molecules by the first principle methods. A brokensymmetry (BS) (or an unrestricted: U) method approximately but easily corrects the static correlation at the lower computational costs. However the BS method involves a serious problem called a spin contamination error (SCE). For the problem, our group has proposed a spinprojection method to eliminate the SCE from the energy derivatives based on Yamaguchi’s approximate spin projection (AP) procedure [14]. By the AP method, one can optimize the geometry of the biradical systems without SCE at the costs of the BS level calculations. However one must carry out 6N (N = optimizing atoms) times singlepoint calculations previous to the geometry optimization because it uses a numerical derivative for d<S2>/d R values. In addition, it also requires both the lowspin and the highspin state calculations during the geometry optimization. Therefore the reduction of the computational costs is a problem of the AP method for the optimization of the larger biradical systems such as a binuclear metal complex. In this study, we attempt to combine the AP method and the spinrestricted (R) methods, i.e. twolayer QM/QM’ approach based on ONIOM method. In the method, the effect of the outerligands is included by the restricted method whilst an energy gradient of the core is calculated by the AP method using a reduced (small) model. The detail about the method and results are illustrated in the presentation.
References
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